1. Field of the Invention
The present invention relates to ceramic compounds which undergo martensitic transformation and methods for producing them, and also to highly-tough composite materials consisting essentially of the compounds. In particular, the invention is to propose ceramic compounds capable of exhibiting high toughness through the phenomenon that follows their thermo-elastic martensitic transformation, and inorganic composite materials.
The ceramic compounds and the composite materials of the invention are effectively used in broad fields of artificial bones, artificial teeth, engine parts, gas turbine blades, parts for gas turbines, parts of corrosion-resistant devices, crucibles, parts for ball mills, electric insulating materials, tools, heat-insulating materials, substrates for electronic circuits, sealants, joints, parts for valves, pumps, nozzles, roller guides, ferrules, bearings, etc.
2. Description of the Related Art
As compared with metals and polymer materials, in general, ceramics have higher hardness, better heat resistance and better corrosion resistance and are characterized by their good electric and magnetic properties.
On the other hand, however, the toughness of ceramics is much inferior to that of metals and polymers, and therefore, ceramics are defective in that their use is limited.
In order to overcome the drawback (low toughness) of ceramics, various proposals have heretofore been made. Of those, one technique for increasing the toughness of ceramics which is based on the martensitic transformation of ceramic materials is most widely noticed, since the method for producing the toughened ceramics according to the technique is easy and the technique itself is greatly effective. For example, known is highly-tough zirconia, for which the technique is based on the non-reversible stress-inducing phase transformation mechanism for non-thermo-elastic martensitic transformation. Specifically, according to the technique, tetragonal crystals of zirconia in a high-temperature mother phase are stabilized at a temperature not higher than room temperature, at which the crystals may be cracked, while being subjected to phase transformation into martensitic monoclinic crystals, whereby the crystals are prevented from being more cracked owing to the volume expansion that results from the phase transformation.
However, as being based on the non-reversible stress-inducing phase transformation, the conventional highly-tough ceramics that undergo martensitic transformation are still problematic in that the toughness in the area around the cracks having been formed during the phase transformation into monoclinic crystals is rather lowered. In addition, if they are much damaged continuously for a long period of time, their toughness is gradually lowered as a whole, resulting in that they could no more have high toughness.
As so mentioned hereinabove, the conventional highly-tough ceramics are based on the phase transformation from the high-temperature phase of tetragonal crystals into the stable martensite phase via the semi-stable phase that is even at temperatures not higher than room temperature. However, the phase transformation of that type will occur even when ceramic compounds react with water, and is especially activated at temperatures falling between 200 and 300.degree. C. Therefore, the conventional highly-tough ceramics are further problematic in that they are unstable.
Moreover, at present, alumina is principally combined with the conventional highly-tough zirconia to produce highly-tough composite ceramic materials, since the combination has been proved effective. Therefore, there is still another problem in that the conventional highly-tough zirconia could not be combined with any other ceramics except alumina to produce highly-tough composite ceramic materials.